In recent years, a multiuser multi-input multi-output (MIMO) technique in which data is transmitted simultaneously to multiple user equipment devices (UEs) from a base station that has multiple transmission antennas has been receiving attention. As the multiuser MIMO (hereinafter, “MU-MIMO”) enables data transmission to multiple UE at the same time using the same frequency, throughput of a wireless communication system can be improved.
In the MU-MIMO, to prevent data to multiple UE from interfering with each other, methods such as zero forcing in which a transmission signal is multiplied by a transmission weight and block diagonalization can be applied. The transmission weight is a weight to adjust the phase and the amplitude of a transmission signal, and by determining the transmission weight according to a channel between the base station device and UE, transmission signals to multiple UE are orthogonalized to each other, thereby reducing the interference.
Non-Patent Document 1: Q. H. Spencer, A. L. Swindlehurst, M. Haardt, “Zero-Forcing Methods for Downlink Spatial Multiplexing in Multiuser5 MIMO Channels”, IEEE Trans. Signal Process, vol, 52, no. 2, pp. 461-471, February, 2004
However, for example, when UE moves, or when a radio wave scattering object around the UE moves, the channel between the base station device and the UE varies every moment by fading. Therefore, the transmission weight optimal to the UE also varies by the variation of the channel, and such a problem arises that the inter-user interference is not reduced sufficiently. That is, because the states of the channel differ from each between when the transmission weight is calculated and when transmission data is actually been transmitted, there is a case where a transmission beam according to the transmission weight is not the optimal one at the time when the transmission data is transmitted.
Specifically, for example, as depicted in FIG. 16, a case in which a transmission beam for UE 20a expressed by a curve B in the drawing is formed when a base station device 10 transmits data to UEs 20a and 20b at the same time is explained. In FIG. 16, a peak of the curve B indicates a direction of a high beam gain, and a valley of the curve B indicates a direction of a null at which the beam gain is 0.
As depicted in the drawing, in the transmission beam for the UE 20a, the beam gain is high in the direction of the UE 20a. On the other hand, not to interfere the UE 20b, a null is directed to the UE 20b in the transmission beam for the UE 20a. However, the UE 20b moves during a period from when the transmission weight for the UE 20a is formed until when the transmission beam is actually formed. As a result, when the transmission beam is actually formed and a signal is transmitted, the UE 20b is not positioned at the direction of a null of the transmission beam for the UE 20a, and the signal to the UE 20a interferes the UE 20b. That is, the orthogonalizing between the UEs by the transmission weight is collapsed, and the inter-user interference occurs. As a result of occurrence of the inter-user interference, the throughput of the wireless communication system is reduced.